Injection of high molecular weight materials such as polymers into the boundary layer of a fluid flow has been shown to reduce skin friction drag significantly for both vessels moving relative to water and for pipeline applications. The large polymer molecules interact with the turbulent activity in the near-wall region, absorbing energy and reducing the frequency of burst (high energy fluid moving away from the wall) and sweep (low energy fluid replacing the high energy fluid in the near-wall region) cycles. The reduced burst frequency results in less energy dissipation from the wall and can result in skin friction drag reductions up to 80%. Experiments have shown that the efficacy of polymer molecules for drag reduction is closely related to their molecular weight, their location in the boundary layer, and the degree to which they have been stretched, or xe2x80x9cconditionedxe2x80x9d.
In the past, polymer mixture ejectors have been simple slots that ejected a mixture/solution of polymer and a fluid at an angle to the wall. To attain high drag reduction for a reasonable distance downstream with this ejection approach, large quantities and high concentrations of polymers must be ejected in order to flood the entire boundary area, creating a xe2x80x9cpolymer oceanxe2x80x9d effect. The high polymer consumption rates of these systems have made them impractical for many drag reduction applications.
Fluids containing other substances than high molecular weight materials, e.g. microbubbles, surfactant, etc., have also been used as additives in prior art attempts to reduce surface friction drag. These, however, each require very large amounts of additive.
To be useful for practical applications, a more efficient method for ejecting for drag reduction needs to be devised.
This invention enables the efficient ejection of fluid mixtures/solutions into the near-wall region of a boundary layer of a fluid flow. The ejector of the present invention has, as a first object of the invention, to condition the polymer prior to ejection so that drag reduction occurs almost immediately following ejection. A second object of the invention is to release a drag reducing substance only into the boundary layer region, where it can provide the greatest drag reduction. A third object of the invention is to retain the drag reducing substance in the near-wall region of the boundary layer, the most effective region for drag reduction, as long as possible. The drag reducing substance may comprise a mixture/aqueous solution of high molecular weight polymer, surfactant, gas microbubbles, or any combination thereof.
The ejector system of the present invention preconditions a polymer mixture/solution for improved drag reduction performance using a unique arrangement of flow area restrictions, as well as by employing dimples, grooves and elastomeric materials. The dimples, grooves and flow area restrictions are sized relative to one another and to the Reynolds number of the flow for optimal polymer molecule conditioning (lengthening, unwinding, or stretching) so as to provide optimal drag reduction after ejection into the fluid flow. In addition, the ejector of the present invention uses a new approach to structuring the flow in order to reduce migration/dissipation of the drag reducing substance away from the near-wall region. This is achieved by a unique system of slots, each having a carefully designed surface curvature and surface features which establish a duct-like system of longitudinal (i.e., in the direction of the flow) Gxc3x6rtler vortices. Gxc3x6rtler vortices are formed by the centrifugal effect of a fluid flow that is given angular velocity by a concave surface. The duct-like system of Gxc3x6rtler vortices formed by the present invention mimic the spacing of naturally occurring quasi-longitudinal vortex pairs in the boundary layer, but are paired in the opposite orientation. The pairing of naturally occurring quasi-longitudinal vortex pairs is such that they migrate from the wall and are believed to contribute to the development of bursts and sweeps that account for a large portion of hydrodynamic drag. The vortices created by the present invention pair, such that the pressure differentials they create cause the vortices to remain near the wall. This advantageously causes the drag reducing substance that has been ejected into the boundary layer to remain in the near-wall region.